Method and apparatus for measuring birefringent particles

ABSTRACT

A method and apparatus for measuring birefringent particles is provided comprising a source lamp, a grating, a first polarizer having a first transmission axis, a sample cell and a second polarizer having a second polarization axis. The second polarizer has a second polarization axis that is set to be perpendicular to the first polarization axis, and thereby blocks linearly polarized light with the orientation of the beam of light passing through the first polarizer. The beam of light passing through the second polarizer is measured using a detector.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. application Serial No.60/181,959 filed Feb. 10, 2000, and PCT Application No. PCT/USO1/04440,filed Feb. 8, 2001, the contents of which are hereby incorporated byreference in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] The invention described and claimed herein was made in partutilizing funds supplied by the United States Department of Energy undercontract No. DE-AC03-76SF000-98 between the U.S. Department of Energyand the Regents of the University of California. The government hascertain rights to the invention.

BACKGROUND OF THE INVENTION

[0003] Field of the Invention

[0004] Particles and dissolved material produced by marine biota in theupper ocean subsequently sinks and becomes remineralized in deeperwaters. This is commonly referred to as the “biological pump”. Thetransport of biogenic carbon from the surface to deep ocean, asparticles and dissolved forms of both oceanic and inorganic material isa fundamental component of the marine carbon cycle and plays a criticalrole in regulating the level of CO₂ in the Earth's atmospheres. Modelsimulations of the preindustrial global carbon cycle indicate thatatmospheric CO₂ concentrations would be approximately 60% higher in theabsence of marine biota. The potential for the oceanic uptake ofanthropogenic CO₂ to change significantly as a result of climate-inducedmodifications to the biological pump, e.g. increased productivity due towarmer temperatures is an issue of key importance to the study of globalclimate change.

[0005] Our understanding of the biological pump is severely limitedbecause conventional ship-based sampling methods, e.g. collectingparticles by filtration using rosette-mounted bottles or large volume insitu filtration, cannot adequately capture the spatial and temporalvariability of biomass and carbon species in the ocean. New technologyand sampling techniques are necessary to advance the current state ofknowledge regarding the functioning of the biological pump and itsconsequences for global carbon cycling. Accordingly the inventors haveintensively investigated methods and apparatuses for the qualificationand quantification of particulate matter. In particular, describedherein are techniques for the investigation of particulate inorganiccarbon. More particularly, described herein are techniques and equipmentfor the investigation of particulate inorganic carbon in seawater.

[0006] Particulate inorganic carbon (PIC) in seawater comprises biogenicparticles of calcium carbonate (CaCO₃). PIC occurs as both calcite andaragonite polymorphs of CaCO₃ in the marine environment, ranging inconcentration from less than 0.01 μmol CaCO₃ L⁻¹ in deep ocean waters toover 30 μmol CaCO₃ L¹ in open ocean surface waters during phytoplanktonblooms.

[0007] The formation of PIC at seawater pH follows the general reaction:

2HCO₃ ⁻(aq)+Ca²⁺(aq)<====>CaCO₃(s)+CO₂(aq)+H₂O (l)  (1)

[0008] From Equation 1, it is evident that PIC formation results in anet reduction of total dissolved inorganic carbon species (CO₂, H₂CO₃,HCO₃ ⁻ and CO₃ ²⁻, collectively referred to as ΣCO₂) and contributes tothe flux of sinking particles that transport carbon from the surface todeep ocean (i.e., the biological pump). But PIC formation also decreasesalkalinity and increases CO₂ in surface marine waters, thereby reducingthe capacity of the ocean for taking up atmospheric CO₂. While it isclear that PIC plays an important role in marine carbon cycling, muchremains unknown about the processes governing its formation, transportand remineralization. Central to the understanding of PIC in marinecarbon cycling is the ability to accurately and precisely measure PIC aswell as other carbon system compounds and particles. Accordingly theinvention described herein provides for a method of measuringbirefringent particles, particularly suspended PIC.

[0009] U.S. Pat. No. 5,993,640 describes a method of measuring the CaCO₃content of a suspension by injecting an acid into the suspension andmeasuring the change in pH.

[0010] U.S. Pat. No. 5,001,070 discloses a method of determining thetotal carbonate content in a fluid by an electrochemical method.

[0011] U.S. Pat. No. 4,683,211 describes a method of measuring theconcentration of CaCO₃ in a slurry by reacting the CaCO₃ with an acid,blowing a known flow rate of air into the slurry, and measuring the flowrate of the mixed gas and the amount of sampled slurry.

BRIEF SUMMARY OF THE INVENTION

[0012] Numerous investigations have been undertaken by the inventors toprovide for a sufficient method of measuring particulate matter.Particularly, the inventors have investigated methods for determiningPIC concentrations in seawater based on the optical property ofbirefringence. Birefringence refers to the ability of a mineral crystalto split an incident beam of linearly polarized light into two beams ofunequal velocities (corresponding to two different refractive indices ofthe crystal) which subsequently recombine to form a beam of light thatis no longer linearly polarized. The extreme birefringence of CaCO₃makes it appear to light up when viewed through crossed polarizers. Theextreme birefringence of calcium carbonate (CaCO₃) relative to othermajor components of marine particulate matter provides a basis formaking optical in situ measurements of particulate inorganic carbon(PIC) in seawater. Because CaCO₃ particles dominate the mineral fractionof marine particulate matter and are much more birefringent than othermajor types of inorganic particles, it is expected that PIC will be thedominant source of any birefringence signal obtained from seawater.

[0013] Thus the invention described herein provides for a method ofmeasuring birefringent particulate matter and an apparatus foraccomplishing the method. Particularly, the invention provides for amethod of measuring suspended particulate matter, and more particularlythe invention provides for a method of measuring suspended PIC inseawater and an apparatus for accomplishing the method.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a schematic diagram of a spectrophotometer configured tomeasure samples in accordance with the invention.

[0015]FIG. 2 shows the transmission of light through parallel andcrossed polarizers by calcite (CaCO₃) and two non-birefringent minerals(NaCl and amorphous SiO₂).

[0016]FIG. 3 is a schematic diagram of one possible design for an insitu optical PIC sensor.

[0017]FIG. 4 is a graph showing the linear relationship betweenabsorbance and total suspended material (TSM).

[0018]FIG. 5 is a graph of absorbance at 660 nm for serial dilutions ofthe pure calcareous sediment and diatomaceous earth suspensions. Thesolid lines indicate least-squares linear regressions of absorbance onTSM for the two suspensions.

[0019]FIG. 6 shows absorbance at 660 nm for serial dilutions of thesuspensions containing mixtures of calcareous sediment and diatomaceousearth. The solid lines indicate least-squares linear regressions ofabsorbance on TSM for the different suspensions. The observed slopesagree with values calculated from the CaCO₃ content of the mixtures andthe slope values of the pure calcareous sediment and diatomaceous earthsuspensions.

[0020]FIG. 7 shows birefringence for the serial dilutions of thesuspensions containing mixtures of calcareous sediment and diatomaceousearth. Data obtained for the pure calcareous sediment and diatomaceousearth suspensions are also plotted for comparison. The two purecalcareous sediment samples with much higher PIC concentrations than therest of the samples (1210 and 1820 μmol CaCO₃ L⁻¹) do not appear on thisgraph.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Our understanding of PIC cycling has been severely limited byconventional ship-based sampling techniques—i.e., chemical analyses ofparticulate material filtered from seawater. This method of samplingcannot adequately assess the factors governing spatial and temporalvariability of PIC in the oceans.

[0022] The invention described herein measures particulate matter,particularly suspended particulate matter and more particularlysuspended particulate inorganic carbon in seawater. The methodcontemplated by the inventors involves placing polarizing means in frontof a source lamp and also in front of a detector and orienting the twopolarizing means such that their transmission axes are perpendicular toeach other (i.e., transmission of incident light front a source lamp isminimized.) Various amounts of particles were suspended in water andplaced in a sample cell between the two polarizers. A strongly linearrelationship was observed between the amount of particles in suspensionand the amount of transmitted light reaching the detector. Analyses ofsuspensions containing diatomaceous earth and mixtures of diatomaceousearth and CaCO₃ indicate that only minor interference results from thepresence of non-birefringement particles.

[0023] By “optical sensor” it is meant a device that responds to aphysical stimulus involving light and transmits a resulting impulse. Anyoptical sensor known in the art may be used with this method.

[0024] By “particulate” it is meant discrete fragments of matter, i.e.particles. Particle or particulate is meant to include a single particleor a plurality of particles or particulates.

[0025] By “inorganic carbon” it is meant carbon contained in compoundsnot classified as organic.

[0026] “Calcite” is meant to include the polymorph of the mineralcalcium carbonate having rhombohedral structure.

[0027] “Aragonite” is meant to include the polymorph of the mineralcalcium carbonate having orthorhombic structure.

[0028] By “extreme birefringence” it is meant that the birefringence ofparticulate inorganic carbon (i.e. calcite and argonite) is at leastfive times greater than the birefringence of other types of particulatematerial typically encountered in seawater.

[0029] By “polarizer”, it is meant any optical device that transmitslight with electric field vectors restricted to a single plane. It iscontemplated that any polarizing technique known in the art will besuitable for the instant invention.

[0030] By “beam of light” is meant light of any wavelength.

[0031] By “cross polarized light” it is meant light that is transmittedthrough two polarizers whose transmission axes are oriented at rightangles with respect to each other.

[0032] “Measuring” is meant to include qualitative and/or quantitativeanalysis.

[0033] “Medium” is meant to include any medium where particles are foundin suspension, including homogeneous or homogeneous mediums.

[0034] The present invention will be more readily understood following adescription of the method in conjunction with the Figures. In FIG. 1, asource lamp (1) is used to generate a source light. It is contemplatedthat any source lamp of light may be used. A grating (2) may be used togenerate monochromatic, unpolarized light. It is to be understood thatthe invention does not require a narrow selection of wavelengths, ratherit is desired that the wavelengths of the light source are in the rangeof effectiveness of the polarizer. A spectrophotometer can be used toselect light at a wavelength where a particular kind of polarizer iseffective. The monochromatic, unpolarized light is passed through afirst polarizer (3) having a first transmission axis, resulting in abeam of light being linearly polarized. This beam of light is allowed tocontact the sample. The sample may be birefringent particles or asuspension thereof. It is particularly preferred that the sample be asuspension of CaCO₃ in seawater. However, the method and apparatusdescribed herein is designed and suitable for the analysis of anyspecies exhibiting birefringence. This would include, but is not limitedto a pipeline slurry and particles on a plain surface. By allowing thebeam of light to contact the sample, it is contemplated that the lightmay indeed pass through the sample, which sample may be in a sample cell(4). However, it is contemplated that the light from the first polarizeronly need to interact with the birefringent sample such that thebirefringent sample splits the beam of light, removing the linearpolarization. Light having interacted with the birefringent sample isthen passed through a second polarizer (5), having a second polarizationaxis. The second polarizer has a second polarization axis that is set tobe perpendicular to the first polarization axis, and thereby blockslinearly polarized light with the orientation of the beam of lightpassing through the first polarizer. What is actually measured is lightpassing through the first polarizer that has had its linear polarizationremoved by interaction with the birefringent particles in the sample.Light having interacted with the birefringent particles is no longerlinearly polarized and therefore can pass through the second polarizer.The beam of light passing through the second polarizer is measured usinga detector (6). Any detector capable or measuring light is suitable. Theamount of light reaching the detector is proportional to the amount ofbirefringent sample in the cell. This concept is readily understood withreference to FIG. 2. If the polarizer axes are parallel to one another,all of the light passing through the sample cell will pass through thesecond polarizer; this includes light impinging on birefringentparticles and non-birefringent particles. If the polarizer 1 andpolarizer 2 axes are crossed (i.e., oriented at a 90 degree angle), onlylight that interacts with the birefringent particles will pass throughthe second polarizer.

[0035] The invention also contemplates a method and apparatus forperforming both an in situ and not in situ analysis of birefringentparticles, particularly PIC suspended in seawater. FIG. 3 displays aschematic of one possible design for an in situ PIC analyzer. The systemis contained in a pressure housing (1). A power supply (2) providespower. A collimated light source (3) provides a beam of light. Polarizer1 (4) is a first polarizer having a first transmission axis andPolarizer #2 (5) is a second polarizer having a second transmissionaxis. These polarizers function as described above. A detector (6)measures the output from the second polarizer and the data is recordedby a data recorder (7). It is contemplated that any apparatus thatemploys two polarizers having crossed transmission axes will besufficient to accomplish the method of this invention. Any light source,detector and data recorder known in the art will work with thisinvention. The path length between polarizers is readily determinable byone having ordinary skill in the art using parameters routinely used inanalytical instrumentation. Generally a path length of greater than zerocm up to 100 cm will be sufficient, but longer path lengths arecontemplated, depending on the end use. The brightness of the collimatedlight source is readily determined by one having ordinary skill in theart depending on the desired application. It is also contemplated thatthe apparatus be can be deployed in a marine environment, autonomously,i.e. without human interaction.

[0036] In the example described herein, a grating monochrometer is used.It is to be understood that any means for selecting a wavelength orrange of wavelengths may be used, a non-limiting example of which is aselector or filter.

[0037] The apparatus can be designed such that the spatial dimensionsand power requirements of the sensor are appropriate for long-termdeployment on autonomous oceanographic platforms in addition to standardship based deployment from a cable.

[0038] Methods

[0039] The following method is described in conjunction with PICsuspended in seawater. However it is to be understood that the inventiondescribed herein can be used for any particles exhibiting birefringence,either in suspension or not.

[0040] Preparation of Sample Suspensions

[0041] Suspensions were prepared from two sources of solid material:calcareous marine sediment collected from a site in the EquatorialPacific (0.95° N, 138.95° W, water depth=4287 m); and commerciallyavailable powdered diatomaceous earth (i.e., a source ofnon-birefringent amorphous SiO₂). The calcareous sediment was rich incoccoliths but also contained a significant amount of calcite fromlarger foraminifera shell fragments. In order to isolate the fraction ofsmaller particles that would most readily remain in suspension, thefollowing settling procedure was applied. Roughly 0.06 g of eachmaterial was added to a separate polyethylene bottle containing 120 mlof saturated CaCO₃ solution (used to minimize dissolution of CaCO₃). Thebottles were placed in an ultrasonic bath for 10 minutes to break up anylarge aggregates of solid material and then shaken vigorously to suspendthe particles in solution. The bottles were allowed to sit undisturbedfor 30 minutes, at which point the upper 100-ml portion of each solutionwas collected for further use and the remainder discarded. Approximately29% and 41% of the original amounts of calcareous sediment anddiatomaceous earth, respectively, were recovered by this procedure.

[0042] After fractionation, a 10-ml aliquot of each suspension waspassed through a 0.4-μm polycarbonate membrane filter. The filters weredried and weighed to determine total suspended material (TSM)concentrations (Table 1). The filters were leached overnight in 2% HNO₃and the leachate was analyzed for Ca by inductively coupled plasmaatomic emission spectroscopy; PIC concentrations were determined for thesuspensions as total acid-leachable Ca (Table 1). In the case of thediatomaceous earth suspension, it is likely that all of the Ca was notpresent as CaCO₃ and the reported PIC concentration should therefore beinterpreted as an upper limit. The two pure suspensions were combined indifferent proportions and diluted with saturated CaCO₃ solution to givea series of mixed suspensions with varying ratios (by weight) ofcalcareous sediment to diatomaceous earth (Table 2). TABLE 1 Compositionof the pure calcareous sediment and diatomaceous earth suspensions. TSMPIC % CaCO₃ (mg ml⁻¹) (μmol CaCO₃ L⁻¹) (by weight) calcareous sediment0.192 1820 95% diatomaceous earth 0.250 1.97 <1%

[0043] TABLE 2 Composition of the mixed suspensions prepared from thepure calcareous sediment and diatomaceous earth suspensions. CalcareousSediment/ Diatomaceous Earth TSM PIC % CaCO₃ Ratio (by weight) (mg ml⁻¹)(μmol CaCO₃ L⁻¹) (by weight) 1:0.5 0.096 613 64% 1:1 0.126 602 48% 1:20.188 604 32% 1:10 0.243 228 9% 1:50 0.248 65.5 3%

[0044] Particle size distributions were determined for the purecalcareous sediment and diatomaceous earth suspensions using a CoulterMultisizer II equipped with a 30-μm aperture. Particle diameters rangedfrom 0.74 μm (the smallest size detectable) to 9.1 μm in the calcareoussediment suspension and to 9.8 μm in the diatomaceous earth suspension.The size distributions for both suspensions had similar shapes and wereskewed towards the smaller-sized particles. Particles between 1 and 2 μmin diameter (i.e., typical coccolith size) comprised a greaterproportion of the calcareous sediment suspension than the diatomaceousearth suspension. The difference between the total numbers of particles(and the total volumes they occupy) in the two suspensions is greaterthan expected based solely on the difference in their TSM values. Thisreflects the higher density of the calcareous material produced bycoccolithophores and foraminifera relative to the more open-structured,siliceous material produced by diatoms.

[0045] Spectrophotometer Analyses

[0046] All analyses were performed on an Amersham Pharmacia BiotechUltrospec 3000 Pro benchtop spectrophotometer equipped with a 1-cm pathlength optical silica sample cell. The spectrophotometer wavelength wasset at 660 nm to match the red LED's commonly used in marinetransmissometers (experiments conducted at different wavelengths yieldedsimilar results). Transmittance (7) was measured and used to calculateabsorbance (A) according to the standard relationship:

A=−log ₁₀ T  (2)

[0047] To measure the birefringence of particles in suspension, thespectrophotometer was modified by installing a pair of Corning Polarcorlinear polarizers. Because light from the source lamp was alreadypartially polarized by the grating monochrometer, it was possible tomaximize the intensity of the fully polarized beam incident upon thesample cell by rotating the polarizer between the source lamp and samplecell. The polarizer between the sample cell and the detector was rotateduntil the transmission of polarized light from the incident beam wasminimized—i.e., the polarizers were crossed. Extinction ratios between1.0×104 and 1.2×104 were typically achieved. To enable measurement ofsmall signals above a near-zero background, the detector gain was set toapproximately 275. The birefringence signal is reported as the ratio ofthe radiant power of the light reaching the detector (corrected forgain) to the radiant power of the light incident upon the front face ofthe sample cell.

[0048] Samples ranging in PIC between 12.1 and 1820 μmol CaCO₃ L⁻¹ wereprepared by serial dilutions of each of the pure and mixed suspensions.Aliquots of the suspensions (ranging in volume from 0.02 to 3 ml) wereadded to the sample cell and diluted to approximately 3 ml total volumewith saturated CaCO₃ solution. The contents of the cell were agitatedwith a pipette before each analysis. Data were acquired digitally for 60seconds at a rate of approximately 1 Hz and an average value wascalculated.

[0049] Samples were prepared and analyzed separately for theconventional (i.e., non-polarized) transmittance and birefringencemeasurements. Each analytical run consisted of 17-31 samples and lasted1-2 hours. A reference cell containing particle-free, deionized waterwas run as a blank between every 5-10 samples. Data for the samples wereblank-corrected by subtracting values linearly interpolated from themeasured blanks. Samples prepared from the pure calcareous sedimentsuspension were analyzed during two separate runs performedapproximately three weeks apart. Samples prepared from the purediatomaceous earth suspension and the mixed suspensions were allanalyzed during the same run.

[0050] Results

[0051] Pure Calcareous Sediment and Diatomaceous Earth

[0052] Absorbance readings for the blanks were less than 0.001. Linearrelationships were observed between absorbance and TSM for the serialdilutions of the pure calcareous sediment and diatomaceous earthsuspensions (FIG. 4). The slope for the calcareous sediment data (2.50ml mg⁻¹) was more than twice as high as the slope for the diatomaceousearth data (1.18 ml mg⁻¹; Table 3). TABLE 3 Summary of least-squareslinear regressions for the serial dilutions of the pure calcareoussediment and diatomaceous earth suspensions. Calcareous DiatomaceousSediment Earth Absorbance number of samples 12 5 slope (ml mg⁻¹) 2.4961.182 standard error 0.017 0.004 intercept −0.0017 −0.0006 standarderror 0.0013 0.0004 correlation coefficient (r²) 0.999 1.000Birefringence Signal (initial linear response range—i.e., PIC < 450 μmolCaCO₃ L⁻¹) number of samples 9 — slope (L μmol⁻¹) 5.37 × 10⁻⁷ — standarderror 7.94 × 10⁻⁹ — intercept −9.03 × 10⁻⁷  — standard error 1.58 × 10⁻⁶— correlation coefficient (r²) 0.998 —

[0053] The birefringence signal for the blanks ranged from 4.78×10⁻⁵ to5.06×10⁻⁵ and drifted 1-3% over the course of an analytical run (Table4). TABLE 4 Birefringence signal for replicate samples and blanks. PIC(μmol Raw Blank-Corrected Standard Error CaCO₃ BirefringenceBirefringence For 60-second L⁻¹) Signal Signal Acquisition Blank 4.777 ×10⁻⁵ 0 4.3 × 10⁻⁸ 4.789 × 10⁻⁵ 0 5.1 × 10⁻⁸ 4.809 × 10⁻⁵ 0 4.6 × 10⁻⁸4.814 × 10⁻⁵ 0 5.4 × 10⁻⁸ 4.815 × 10⁻⁵ 0 5.7 × 10⁻⁸ mean 4.801 × 10⁻⁵ —std dev 1.683 × 10⁻⁷ — Rel std 0.35% — dev Blank 5.062 × 10⁻⁵ 0 5.7 ×10⁻⁸ 5.009 × 10⁻⁵ 0 4.4 × 10⁻⁸ 4.975 × 10⁻⁵ 0 6.4 × 10⁻⁸ 4.901 × 10⁻⁵ 05.6 × 10⁻⁸ mean 4.987 × 10⁻⁵ — std dev 6.729 × 10⁻⁷ — rel std dev 1.35%— 30.3 6.65 × 10⁻⁵ 1.66 × 10⁻⁵ 2.2 × 10⁻⁷ 6.58 × 10⁻⁵ 1.62 × 10⁻⁵ 7.9 ×10⁻⁸ 6.53 × 10⁻⁵ 1.60 × 10⁻⁵ 1.5 × 10⁻⁷ mean 6.59 × 10⁻⁵ 1.63 × 10⁻⁵ stddev 5.89 × 10⁻⁷ 2.80 × 10⁻⁷ rel std dev 0.89% 1.72% 303 2.21 × 10⁻⁴ 1.71× 10⁻⁴ 3.2 × 10⁻⁷ 2.22 × 10⁻⁴ 1.72 × 10⁻⁴ 2.3 × 10⁻⁷ 2.12 × 10⁻⁴ 1.62 ×10⁻⁴ 5.6 × 10⁻⁷ mean 2.18 × 10⁻⁴ 1.69 × 10⁻⁴ std dev 5.68 × 10⁻⁶ 5.46 ×10⁻⁶ rel std dev 2.60% 3.24% 1210 5.72 × 10⁻⁴ 5.22 × 10⁻⁴ 4.0 × 10⁻⁷5.75 × 10⁻⁴ 5.25 × 10⁻⁴ 3.0 × 10⁻⁷ 5.78 × 10⁻⁴ 5.29 × 10⁻⁴ 3.0 × 10⁻⁷mean 5.75 × 10⁻⁴ 5.25 × 10⁻⁴ std dev 3.08 × 10⁻⁶ 3.41 × 10⁻⁶ rel std dev0.54% 0.65%

[0054] The blank-corrected birefringence signals for the samples wereless than 1×10⁻³—i.e., <0.1% of the radiant power of the incident lightfrom the spectrophotometer source lamp. A positive relationship wasobserved between birefringence and PIC for the serial dilution of thepure calcareous sediment suspension (FIG. 6). The response initiallyfollowed a linear trend (slope=5.37×10⁻⁷ L μmol⁻¹; Table 3), falling offas PIC increased above 450 μmol CaCO₃ L⁻¹. The calcareous sediment dataobtained on different days fell along the same trend, demonstrating theconsistency of the measurements between different analytical runs. Nodetectable signal was observed for the samples prepared from the purediatomaceous earth suspension.

[0055] The standard error of the birefringence signal over the 60-seconddata acquisition period was 2-8 times higher for the samples than forthe blanks; the standard error was generally higher at the upper end ofthe analytical range and dropped considerably as PIC decreased below100-200 μmol CaCO₃ L⁻¹ (Table 4). This indicates the greater variabilityinherent in a suspension of moving particles relative to particle-freesolution. The minimum detectable birefringence signal (defined as threetimes the standard error of the blank signal over the 60-secondacquisition period) ranged from 1.30×10⁻⁷ to 1.93×10⁻⁷. Based on theinitial linear relationship between birefringence and PIC observed forthe calcareous sediment (FIG. 5, Table 3), the minimum detectablebirefringence signal is produced by values of PIC between 0.24 and 0.36μmol CaCO₃ L⁻¹. This represents the lowest detection limit possible fora 60-second data acquisition period given the intrinsic signal noise ofthe modified spectrophotometer.

[0056] The precision of the method (2σ for triplicate analyses of the30.3, 303 and 1210 μmol CaCO₃ L⁻¹ samples) ranged from 1.3% to 6.5% ofthe blank corrected signal (Table 4). These values are a measure ofuncertainty due to both signal noise and procedural error (i.e.,instrument drift, pipetting inaccuracy, etc.). For the 30.3 μmol CaCO₃L⁻¹ sample, the standard deviation of the triplicate analyses is of thesame magnitude as the standard error of the average birefringence signalover the 60-second acquisition period for the individual analyses. Theuncertainty in the measurement is therefore primarily due to signalnoise at this concentration of PIC. For the 303 and 1210 μmol CaCO₃ L⁻¹samples, the standard deviation of the triplicate analyses is roughly anorder of magnitude higher than the standard error of the averagebirefringence signal over the 60-second acquisition period for theindividual analyses. This indicates that most of the uncertainty in themeasurement at higher concentrations of PIC is due to procedural error,with only a relatively small contribution from signal noise.

[0057] To verify that the birefringence signal was primarily due to thepresence of CaCO₃ particles, undiluted pure calcareous sedimentsuspension was acidified and reanalyzed. Upon addition of HNO₃ to thesample cell, absorbance dropped from 0.474 to 0.013, in agreement withthe 95% CaCO₃ content determined for the calcareous sediment material.Birefringence dropped from 6.42×10⁻⁴ to 2.77×10⁻⁶, indicating thatvirtually all (99.6%) of the signal was due to CaCO₃. The small residualsignal is consistent with the expected presence of clay minerals andother weakly birefringent material in the calcareous sediment.

[0058] Mixtures of Calcareous Sediment and Diatomaceous Earth

[0059] Absorbance and TSM were linearly related for the serial dilutionsof each of the suspensions containing mixtures of calcareous sedimentand diatomaceous earth (FIG. 6). The slopes ranged from 1.12 to 2.00 mlmg⁻¹ (within the limits defined by the slopes for the pure calcareoussediment and diatomaceous earth suspensions) and increased in proportionto the CaCO₃ content of the suspensions.

[0060] Birefringence and PIC were positively correlated for the serialdilutions of the mixed suspensions with CaCO₃ content≧9% (FIG. 7). Aswas observed for the pure calcareous sediment suspension, the responsefell off as PIC increased. Sensitivity decreased as the relativeproportion of CaCO₃ in the suspensions decreased, dropping approximately2-fold between the pure calcareous sediment suspension and the 9% CaCO₃suspension. No detectable signal was observed for the samples preparedfrom the 3% CaCO₃ suspension, with the particular apparatus setup usedfor this example. It is contemplated that CaCO₃ samples well below 3%are readily detectable by an apparatus change or data handling techniquereadily available to one having ordinary skill in the art.

[0061] It is to be understood that in the foregoing example, thespectrophotometer was operated at a gain setting far above its normaloperating range. In developing an in situ or ex situ apparatus formeasuring birefringent samples, especially particles in suspension, thesource lamp and detector apparatus and other instrument parameters, aswell as data reduction techniques, are able to be optimized by onehaving ordinary skill in the art without undue experimentation with thespecific intent of obtaining the most stable birefringence signal aspossible.

[0062] The foregoing description is intended primarily for purposes ofillustration. Although the invention has been described with respect toan exemplary embodiment thereof, it should be understood by thoseskilled in the art that the foregoing and various other changes,omissions, and additions in the form and detail thereof may be madetherein without departing from the spirit and scope of the invention.

What is claimed is:
 1. A method of analyzing a birefringent samplecharacterized by, generating a first beam of light using a firstpolarizer, said first polarizer having a first transmission axis,contacting the sample with the first beam of light, thereby creating asecond beam of light, passing the second beam of light through a secondpolarizer, said second polarizer having a second transmission axis,measuring the light passing through the second polarizer, where thefirst transmission axis is arranged to be perpendicular to the secondtransmission axis.
 2. The method of claim 1 wherein the birefringentsample comprises a particle.
 3. The method of claim 2, wherein theparticle is suspended in a medium.
 4. The method of claim 3, wherein themedium comprises a liquid.
 5. The method of either of claims 3-4,wherein the medium comprises water or seawater.
 6. The method of claims2-5, wherein the particle comprises CaCO₃.
 7. An apparatus for analyzinga birefringent sample characterized by having a first polarizer having afirst transmission axis, and a second polarizer having a secondtransmission axis, said first polarizer and second polarizer arrangedsuch that light emitted from the first polarizer impinges on the sampleand is subsequently passed through the second polarizer, where saidfirst transmission axis and said second transmission axis areperpendicular to each other.
 8. The apparatus of claim 7, wherein thesample comprises a particle.
 9. The apparatus of claim 8, wherein theparticle is suspended in a medium.
 10. The apparatus of claims 8-9,wherein the particle comprises CaCO₃.
 11. The apparatus of claims 9-10,wherein the medium comprises a liquid.
 12. The apparatus of claims 9-11,wherein the medium comprises water or seawater.
 13. The apparatus ofclaims 8-10, wherein the particle comprises CaCO₃ suspended in water orseawater.